Focused electron beams having a very small focus diameter are used in electron microscopy for generating raster microscopic images of surfaces as well as for generating raster microscopic images of objects which can be transilluminated. In addition, applications for analyzing materials are known such as by means of X-rays generated by the electron beam in the object. Applications are also known in the energy-loss spectroscopy and for three-dimensional structural analysis by convergent diffraction.
The focus diameter of the electron beam on the object is generally defined by the demagnified image of the crossover of the electron source with the aid of several condenser lenses and one objective lens. The aperture of the electron beam is adjusted by a condenser diaphragm. In order to obtain a maximum electron current for a desired probe diameter, the aperture is so selected that no significant widening of the electron probe takes place because of the aperture aberration of the illuminating system. The aperture aberration, especially of the objective lens of a scanning electron microscope, therefore requires an adaptation of the probe aperture to the selected focus diameter.
The convergent diffraction is a microdiffraction method wherein the focus of the electron beam is fixed on the object. A variable adjustment of the aperture (that is, of the angle of convergency) is required for adapting the angle of convergency to the Bragg angle of the crystal and for adjusting different diffraction diagrams (such as the Kossel diagram or Kossel-Mollenstedt diagram).
U.S. Pat. No. 4,626,689 discloses an electron microscope illuminating system wherein the crossover of the electron source is focused on the object via a three-stage condenser and a further electron lens. The third condenser stage is only weakly excited and generates only an imaginary image. The excitations of the four lenses are so selected that different convergence angles can be set with only a single fixed condenser diaphragm with the focus diameter being constant. However, it has been shown that the simultaneous setting of a constant focus diameter and a variable aperture greatly limits the range of variation of the aperture. Even a variation of the aperture by a factor of 1.5 to 2 requires a mechanical exchange of the condenser diaphragm in dependence upon the selection of the diaphragm diameter. This requires a recentering of the diaphragm which has been mounted in exchange.
U.S. Pat. No. 5,013,913 likewise discloses a scanning electron microscope illuminating system having a three-stage condenser and a downstream objective. Here, the third condenser lens generates a real image of the crossover in the input image plane of the objective. The focus diameter of the electron beam can be varied by varying the excitations of the two first condenser lenses which operate as a zoom system. The aperture of the illuminating beam is determined by a condenser diaphragm which is mounted either forward or rearward of the input image plane of the objective lens. Furthermore, several embodiments for adjusting Kohler-type illuminating conditions for an electron microscope are described. A multiple diaphragm is mounted in the proximity of the primary plane of the third condenser stage for adjusting an illuminating field which is independent of the illuminating aperture. The diaphragm aperture of the multiple diaphragm is selected with the aid of one or several beam deflectors and determines here the size of the illuminating field on the object. However, the above U.S. Pat. No. 5,013,913 does not disclose whether and in which way such a multiple diaphragm can be used also for an illuminating system wherein an electron beam is focused on the object and the focus diameter is varied via the excitations of the first two condenser lenses.
European patent publication 0,179,294 discloses an ion-optic illuminating system wherein the crossover of the ion source is focused on the object surface via a two-stage imaging system. A multiple diaphragm is mounted between the two imaging stages. A single opening of the multiple aperture can be selected via beam-deflecting systems which are likewise interposed for varying the illuminating aperture. However, in this system, a variation of the focus diameter and the illuminating aperture independently of each other is not possible because, with a change of the lens excitations, a change also takes place simultaneously with respect to the spacing between the multiple diaphragm and the intermediate image plane and therefore the illuminating aperture.
U.S. Pat. No. 5,051,556 discloses an electron-beam lithography system wherein the electron beam is focused on the wafer via a five-stage demagnification system. A mask with a plurality of component fields is mounted in the plane of tile second imaging stage. The selection of the desired mask array takes place by deflecting and thereafter deflecting back the electron beam out of or into the direction of the optical axis. In such lithography systems, the mask, however, does not operate to adjust a specific illuminating aperture; instead, it operates to impress a specific pattern which is thereafter imaged with reduction on the object.